This invention relates to a weight sensing apparatus and, more particularly, to a weight sensing apparatus suitable for use in a computerized weighing system.
Weighers adapted to weigh articles mechanically are gradually being replaced by weighers which rely upon electronic circuitry. A weighing system using electronic circuitry employs a load cell comprising a load-sensitive element and a strain gauge which is affixed to the load cell. The resistance of the strain gauge changes owing to strain imposed when the strain-sensitive element is subjected to an applied weight.
Load cell structures of the above-described type have already been disclosed in a number of patent publications, e.g., Japanese Patent Application No. 57-187744 and 57-198259, and Japanese Utility Model Application No. 57-161840.
The load cell in a weighing system is adapted to produce an analog signal indicative of the applied weight. The signal is amplified, converted into a digital value by an analog-to-digital converter (referred to as an A/D converter), and the digital value is delivered to a display unit where the value is displayed. With an ordinary weighing apparatus of this kind, weighing elements such as a weighing dish and the load cell proper experience a gradually attenuating oscillation on the order of several score Hertz owing to shock which is sustained when the operator places the articles to be weighed on the weighing dish. This oscillation is transmitted to the load cell, which develops noise as a result. This causes the numerical value displayed by the display unit to fluctuate, with considerable time being required for the value to stabilize. The operator must therefore wait for the oscillations to cease before he can read the value displayed. These oscillations are be referred to as "weighing system oscillation" in the following.
Weighing system oscillation can be reduced by inserting a low-pass filter, which has a very low cut-off frequency on the order of 5 to 10 Hz, in the amplifier circuit that amplifies the analog weight signal produced by the load cell. This makes it possible to cut-off the oscillation frequency to the extent that the operator can read the displayed numerical value only a short time after placing the articles on the weighing dish. However, even stationary structures supporting the load cell, as well as the weighing apparatus proper, experience oscillation with respect to the environment in which the apparatus is installed. Such oscillation derives from ground vibration, as well as from vibration of the building itself, the floor on which the apparatus is set, supporting framework, etc. Such oscillation sustained by the load cell is referred to as "floor oscillation" in the following. Electrical oscillation caused by floor oscillation poses a special problem. Specifically, the abovementioned low-pass filter is capable of eliminating frequencies of 10 Hz or more; frequencies below this value act upon the weight signal. If a low-pass filter having an even lower cut-off frequency is inserted in the amplifier circuit in order to reduce the lower frequencies, too much time will be needed for the displayed value to attain the value indicated by the weight signal. The result will be a slower weighing speed, making it impossible to achieve a high-speed weighing operation.
Computerized weighing systems have been developed in recent years. With a weighing system of this type, a computer is adapted to form a plurality of weight values obtained from a plurality of weighing hoppers into a number of combinations, calculate the total weight of the values in each combination, and obtain a total weight value which is equal or nearest to a predetermined target weight value. Since a computerized weighing system is characterized by the fact that weighing can be performed at high speed, the weight values obtained from the weighing hoppers ought to be applied to the computer immediately after the articles to be weighed are introduced into the hoppers. However, since a computerized weighing system is installed in a factory or the like, the environmental conditions are even worse than those experienced by the ordinary weighing apparatus, and the so-called floor oscillation that acts upon the system is accompanied by supporting frame oscillation that derives from the operation of peripheral equipment. In consequence, the weight values cannot be fed into the computer immediately after articles are charged into the weighing hoppers. This is an impedement to high-speed weighing and makes necessary a troublesome operation in which the floor oscillation characteristic, which differs depending upon the place of installation, is measured on-site, followed by resetting the cut-off frequency of the low-pass filter on the basis of the data obtained from the measurements. The maximum weighing speed at which the weighing system can operate with good weighing precision represents the limit upon the weighing speed achievable at the place of installation.